Persistence of seed‐applied pesticides in soybeans: environmental influences and implications for crop protection

BACKGROUND: Early soybean (Glycine max (L.) Merr.) planting is increasingly adopted in Indiana and the upper Midwestern USA to maximize yield potential, yet seedlings may be more vulnerable to early-season pests and environmental stressors. Seed-applied systemic insecticides and fungicides are widely used to mitigate these risks, but their in-plant concentrations under cold and wet spring soil conditions remain undefined. We quantified thiamethoxam, mefenoxam, fludioxonil, and sedaxane across planting dates, growth stages, and locations. RESULTS: During a 2-year field study at three Indiana locations, soybeans were planted from early April to late May. Tissues were sampled at five growth stages: VE, VC, V1, V2, and V3/V4. Thiamethoxam was detectable in plant tissues up to later vegetative stages (V4) in both early- and late-season plantings, whereas fungicides persisted only to the first trifoliate (V1) stage. Thiamethoxam concentrations were highest at emergence (~30 000-60 000 ng/g), primarily in cotyledons, and declined sharply by V4 (~50 ng/g). Associations between tissue concentrations and planting date or abiotic conditions varied by compound, year, tissue, and growth stage. No uniform planting date effect was observed. CONCLUSION: Seed-applied pesticides were concentrated in early seedling tissues and declined after emergence. No consistent relationships were observed between planting dates or abiotic conditions examined and pesticide residues in soybean tissues, making predictions about alignment between in-plant concentrations and early-season pest risk difficult. These early-season field observations complement a literature base indicating that seed-applied pesticides provide a brief, inconsistent window of protection that does not extend beyond the earliest growth stages.